The regulation of body weight and energy balance involves communication between signals emanating from peripheral al tissues and the brain, most importantly, specific neural groups within the hypothalamus. Leptin is a 16 kD hormone derived principally from adipose cells that plays an essential role in informing the brain of the status of energy stores and energy balance. In the absence of leptin or leptin signaling, obesity and severe neuroendocrine disturbances ensue. In most obese rodents and humans, leptin resistance accompanies obesity, but the molecular basis for this is not understood. An understanding of leptin resistance in disease states requires that the key steps required for leptin action to be initiated in the brain be delineated. These include: the mechanism by which leptin is transported across the blood brain barrier (BBB) to sites of action in the brain; the mechanism of leptin signaling through it's receptors in specific target cells; the mechanisms for limiting or terminating leptin signaling; identifying the neural circuitry through which leptin signals in direct target neurons are transmitted to downstream effected pathways. This proposal will address each of these questions. Aim 1 will address the functional status of short isoforms of the leptin receptor, and the molecular basis for BBB transport of leptin, and will seek explanations for the putative defect in such transport in diet induced obesity and NZO mouse with multigenic obesity. Aim 2 will address the pathways engaged by the dominant signaling form of the leptin receptor, and the mechanisms that limit leptin signaling, especially the molecule SOCS-3. Efforts will be made to characterize the state of leptin signaling, and leptin resistance mechanisms, in diet induced obesity and NZO mice. The role of SOCS-3 in inhibiting leptin signaling will be explored through both viral vector gene therapy and gene targeting/transgenic approaches. Aim 3 will characterize and exploit fetal rat hypothalamic cells and SV40 T antigen immortalized hypothalamic cell lines established from these, as models systems for efficient studies of neural targets for leptin action. These will be used to characterize leptin signaling and leptin resistance in relevant cell types, and to establish in vitro models for defects in central pathways relevant to human disease.